1. Field of the Invention
The invention relates to an apparatus for orbit control of at least two co-located geostationary satellites.
2. Description of the Prior Art
Due to the great variety of disturbing forces from the earth, sun and moon, a position maintenance of geostationary satellites, generally referred to as station keeping, requires a constant orbit control to keep a satellite in the tolerance window allocated to it. Tolerance window means a region which is defined in longitude and latitude around the nominal position above the equator with a magnitude of usually .+-.0.05.degree. to .+-.0.1.degree. in longitude and latitude. In the present case the term orbit control means a control and monitoring of the translational satellite movement.
An orbit control is made more difficult by the increasing practice of operating several satellites in a common tolerance window. Thus, for example, at present Astra 1A and 1B are operated on 19.2.degree. east and there are plans to increase this by Astra 1C and 1D in the years 1993 and 1994. This practice is referred to as co-positioning, that is in English co-location. For directly transmitting satellites the WARC (World Administrative Radio Conference) allocated locations and frequencies in 1977 and made free use of co-location. For example, the position 19.degree. west (.+-.0.1.degree. in longitude and latitude) was allocated to the space stations (satellites) of twelve countries, seven European and five African.
For an orbit control for station keeping, essentially an orbit determination, maneuver planning and a maneuver calibration are to be carried out. These operations must be repeated in a correction cycle from seven to fourteen days over the entire life of a satellite, usually seven to ten years.
In an orbit determination, generally the parameters or orbit elements of a predetermined orbit model describing the satellite motion mathematically are to be determined in such a manner that the angle and range measurements made by one or more ground stations and referred to as tracking data are represented as accurately as possible by the model. Once the orbit elements are known at a reference point of time, referred to as epoch, it is possible to reconstruct or predict therefrom the location coordinates and velocity components, i.e. the state vector, for each desired time section as a function of the time.
Orbit maneuvers mean velocity changes of the satellites which are achieved by activating and deactivating selected satellite-carried thrust jets. Jet arrays generally permit thrust in the north/south and in the east/west direction. The orbit elements then also change in caculable manner. On the basis of the orbit prediction correction maneuvers may be planned in such a manner that certain desired orbit elements are achieved and thus infringements of the tolerance window or close encounters with other satellites are avoided.
By means of a maneuver calibration, from the tracking data of a time interval not only the orbit elements but in addition also thrust components of the maneuvers carried out in this interval are determined and compared with desired values. A maneuver calibration makes it possible to take the deviations found into account during future maneuver planning and thus enable the accuracy to be improved.
The methods used at present for orbit control have in particular the following disadvantages:
1. An orbit control for station keeping is carried out at present in many operational steps which as a rule are effected consecutively by calling up individual computer programs each having a relatively restricted function scope. PA1 2. In the orbit determining process maneuvers necessary for orbit keeping are not taken into account or only inadequately taken into account. For the calibration of the maneuver it is therefore necessary to carry out independent orbit determinations before and after the maneuver and this requires a tracking period of about 2 days before and after the maneuver. As a result, a quick reaction to execution errors or other disturbances is not possible, as would be the more important the higher the number of satellites located in a common window. This also has a detrimental effect on the calibration accuracy and the operational expenditure is made more difficult and increased by the necessity of performing several different program runs and the correspondingly increased administration of the results. PA1 3. The planning of station keeping maneuvers, that is position maintaining maneuvers, is designed for the operation of individual satellites and serves essentially to avoid window infringements. No account is taken of strategies for avoiding close encounters in the event of several satellites in a common window. PA1 4. Known systematic maneuver execution errors cannot be taken into account in the maneuver planning. This results in an uncertainty in the prediction of the satellite location and in the case of co-located satellites makes it necessary to increase the safety distances apart. This in turn means that maneuvers for avoiding dropping below these safety distances have to be carried out more frequently. Poorly calibrated maneuvers have the same effect. PA1 5. Orbit data for a maneuver planning, a station prediction regarding the time variation of the directional angle of the ground antenna to the satellite, a prediction of shadow passage times and time regions in which disturbing effects due to the sun and moon on the infrared earth sensors are possible, etc., are administered in the form of ephemerides. These are lists containing the position and velocity of the satellite in discrete time steps. Due to their extent, such ephemerides require a great deal of memory space and are complicated to handle. A rapid assessment of the physical information content of such ephemerides lists with regard to orbit disturbances, maneuvers, etc., is rather impaired than facilitated by the high redundancy. This applies in particular in unexpected and time-critical situations. PA1 6. An exchange and comparison of orbit data between different control centres is not provided as an integral part of the orbit control process, because the methods used hitherto were developed only for individual satellites. On the other hand the operation of several satellites in a common window by different control centres requires specific interfaces for passing on and receiving orbit information in a data which is tolerant with regard to systematic simulation errors of different orbit control systems. PA1 a) a first function block for orbit determination from the measurements of ground stations determines all parameters which are necessary for calculation of the ephemerides in the form of lists containing position, velocity and other important orbit-relevant quantities in discrete time steps, as functions of the time for a desired time period, and calibration data of executed orbit maneuvers for the co-located satellites and stores them in the orbit database; PA1 b) the second function block for maneuver planning accesses the results stored in the orbit database and calculates therefrom parameters for orbit maneuvers so that a satellite at predetermined dates reaches a fixed desired orbit, the results once again also being stored in the orbit database; PA1 c) with the aid of the orbit and maneuver parameters stored in the orbit database the third function block for monitoring the relative movement calculates the relative distances and velocities, resolved by components, between all co-located satellites for a desired period of time and PA1 d) the fourth function block for predicting specific events and ephemerides employs the results of the other function blocks stored in the orbit database for predicting events such as the passage of a satellite through the earth or moon shadow, a disturbance of an infrared sensor by entrance of the sun or moon into the field of view thereof, a position of the sun near the line of sight of the ground station to the satellite, etc., and to compute a detailed orbit ephemeris. PA1 a) orbit determination and maneuver estimation or maneuver calibration; PA1 b) maneuver planning, PA1 c) monitoring of the relative movement and PA1 d) prediction of specific events.